Add "__STL_NO_SGI_IOSTREAMS" to "Preprocessor definitions". This tells STLPort
to use the native VC6 iostream classes instead of its own (which require additional
configuration steps).

In the same window, add "c:\stlport-4.0\stlport" into "Additional include
directories." This tells the compiler to look here before the VC6 headers
when you include STL headers.

You can then use STLPort with that project. You'll need to repeat this for
each new project that you create. It's also possible to use the STLPort iostreams
library instead of the VC version, but that requires rebuilding the Visual C++
runtime library (which you may not want to do).